Research Of Asynchronous In OFDM Cooperative Communication Systems With Insufficient Cyclic Prefix

As the virtual MIMO technology, Cooperative communication combines the advantages of diversity technology and relay transmission technology, which has been the important technology of the new generation wireless communication standards, making the technology of MIMO practical. However, different from the conventional MIMO technology, the collaboration nodes is distributed in space, which makes the problem of asynchronizaton, so that many techniques used in conventional MIMO system can not be employed directly. OFDM technology is applied in asynchronous cooperative communication to combat the time delays from relay nodes. Relay nodes uses special space-time coding to resist asynchronous interference. When the length of cyclic prefix is not less than time delays, interference caused by time delays can be cancelled completely. However, time delays is not only related to signal bandwidth, but also related to the given environment of system, so the length of CP may not less than time delays, and the cyclic prefix lowers the symbol transmission speed. In order to solve the above problem, this paper provides solutions for asyncronhous interference in cooperative communication with insufficient CP.This paper first studies the main technology in cooperative communication system. Analyze the forwarding mechanism of relay nodes in cooperative communication system and the combining technique for multi-channel signal at receiver, and simulate their performance; Take OFDM communication system as an example studying the theory of OFDM;Meanwhile introduce the principle of space-time coding and the design of distributed space-time coding.Then, the paper studies the simple transmission scheme for asynchronous cooperative communication. The relay nodes adopt space-time coding, which guarantees the structure of space-time code at receiver, using ML for decoding. This can combate the destruction of space-time code from asynchronization, and achieve full diversity gain.Finally, the paper goes further research for OFDM asynchronous cooperative communication with insufficient CP. Firstly, study the time-domain processing scheme, use the property of space-time code in frequency domain and time domain at relay nodes to estimate interference at receiver and remove it. Due to the complexity of time-domain interference cancellation, the paper propose the scheme of non-linear precoding at relays. The simulation not only can eliminate interference of asynchronous, but also can achieve full diversity gain.